Apparatus and method for treating ventricular tachyarrhythmias

ABSTRACT

A system and method for selectively treating a ventricular tachycardia based on sensed atrial and ventricular intervals from the patient&#39;s heart. A detection window of the ten most recent atrial and ventricular intervals are analyzed for the occurrence of either tachycardia or fibrillation. When a majority of the sensed intervals are satisfied, the apparatus starts a duration time interval. Ventricular intervals and atrial intervals are compare, ventricular interval greater than the atrial interval by a bias factor the system delivers tachycardia therapy to the heart. Alternatively, the method withholds tachycardia therapy to the heart when the atrial rate is classified as atrial fibrillation and the ventricular response is unstable.

This application claims the benefit under 35 USC § 119(e) of U.S.Provisional Application No. 60\045212, filed Apr. 30, 1997.

FIELD OF THE INVENTION

The present invention relates generally to implantable pulse generatorsand in particular to implantable cardioverter-defibrillators fortreating ventricular tachyarrhythmias.

BACKGROUND OF THE INVENTION

Implantable cardioverter-defibrillators (ICDs) have evolvedsignificantly since their clinical introduction by Miroski in 1980.Initial ICDs were designed to recognize ventricular fibrillation and todeliver high-energy shocks in an attempt to treat the arrhythmia.However, clinical electrophysiology research indicated that an ICDcapable of recognizing and treating ventricular tachycardias as well asventricular fibrillation was useful for prevention of arrhythmic death.

Subsequent ICD development lead to devices that were able to treatventricular tachycardias with antitachycardia pacing and low-energycardioversion shocks in conjunction with back-up defibrillation therapy.These ICDs monitored the heart rate and the onset of the ventriculararrhythmia from ventricular endocardial signals to determine when theheart was in need either of cardioversion to treat a ventriculartachycardia or of defibrillation to treat ventricular fibrillation.While it was successful in detecting ventricular arrhythmias, the ICDswere unable to reliably discriminate sinus tachycardia and atrialarrhythmias, particularly paroxysmal atrial fibrillation, from malignantventricular rhythms because of the sole reliance on ventricular cardiacsignals to determine the cardiac state. As a result, the ICD mightdeliver inappropriate therapy based on aberrant ventricular signals thathave their origins in an undetected supraventricular tachyarrhythmia,leading to an uncomfortable cardioversion shock being delivered to thepatient.

In an attempt to correct this problem ICDs have been designed with dualchamber sensing capabilities to detect and analyze both ventricular andatrial endocardial signals. This increase in cardiac signal input to theICD has provided an opportunity to determine the origin and the natureof the ventricular tachyarrhythmia, and to reduce the frequency ofinappropriate therapy being delivered to an implant patient. However,while the combination of antitachycardia pacing with low and high energyshock delivery as well as backup bradycardia pacing in ICDs has expandedthe number of clinical situations in which the device may appropriatelybe employed, means of coordinating ventricular and atrial rateinformation in a way that results in a system that effectively andefficiently treats an implant patient is still desired.

SUMMARY OF THE INVENTION

The present invention includes a system and a method for reducingunnecessary treatment of a rapid ventricular rate caused by a conductedatrial fibrillation. The system and method treat ventricular tachycardiaby first sensing and analyzing cardiac signals from both the atrial andventricular chambers. Pathological heart rhythms are detected bydetermining the patient's heart rate, the atrial/ventricularrelationship of the rhythm, the suddenness of onset rate of thearrhythmia, the rate stability of the rhythm, the sustained rateduration of the arrhythmia, and whether the atria are in a state offibrillation. This information is then used to assess the origin of arapid ventricular rate and to determine when and what type ofventricular therapy is to be delivered to the heart. By analyzing theorigin of the rapid ventricular rate prior to delivering ventriculartachycardia therapy, true ventricular tachycardia can be treated quicklyand when ventricular tachycardia is of atrial origin, treatment can bedelayed to determine if will convert naturally.

According to one embodiment of the present invention there is provided asystem including (1) electronic control circuitry within an implantablehousing coupled to implantable ventricular and atrial catheters foridentifying and analyzing cardiac signals in the manner described aboveand for providing electrical energy to the heart to affect sinus rhythmof the heart in the manner described above in response to a signal fromthe electronic control circuitry indicating the occurrence of an atrialarrhythmia.

The system allows the user to partition therapy into a maximum of threeprogrammable tachycardia therapy zones. The system provides tachycardiatherapy to the patient in the form of anti-tachycardia pacing pulses,low energy cardioversion shocks, or high voltage defibrillation shocks.Anti-tachycardia pacing pulses may be adapted to the individual chamber(atrium or ventricle) depolarization rates. A variety of conversionschemes including, fixed or adaptive burst, ramp, scan, and ramp/scan,may be selected. To convert rapid ventricular tachycardia (VT) andventricular fibrillation (VF), the system provides both monophasic andbiphasic, synchronous, defibrillation shocks to the patient

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a cardioverter-defibrillator including asystem according to the present invention that is implanted in the humanbody and is coupled to atrial and ventricular leads implanted in a humanheart;

FIG. 2 is a block diagram of electronic control circuitry included in asystem according to the present invention;

FIG. 3 is a flow diagram of a first portion of the manner in which theelectronic circuitry included in the system operates according to thepresent invention;

FIG. 4 is a flow diagram of a second portion of the manner in which theelectronic circuitry included in the system operates according to thepresent invention;

FIG. 5 is a graph illustrating a determination of the presence orabsence of sudden onset rate of a ventricular tachycardia by the systemaccording to the present invention;

FIG. 6 is a graph illustrating a determination of the stability orinstability of a ventricular tachycardia by the system according to thepresent invention.

FIG. 7 is a chart illustrating an example of the use of the detectionenhancements.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings which form a part hereof and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice and use the invention, andit is to be understood that other embodiments may be utilized and thatelectrical, logical, and structural changes may be made withoutdeparting from the spirit and scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense and the scope of the present invention is defined by theappended claims and their equivalents.

Referring now to FIG. 1 of the drawings, there is shown a system 20including an implantable pulse generator 22 physically and electricallycoupled to an atrial catheter 24 and a ventricle catheter 26, whichsystem 20 may be used in practicing the method according to the presentinvention. The system 20 is implanted in a human body 28 with portionsof the atrial and ventricular catheters 24 and 26 inserted into a heart30 to detect and analyze electric cardiac signals produced by both theatria 32 and the ventricles 34 of the heart 30 and to provide electricalenergy to the heart 30 under certain predetermined conditions to treatventricular tachycardias and ventricular fibrillation of the heart 30.

A schematic of the pulsed generator 22 electronics is shown in FIG. 2.The implantable pulse generator 22 comprises an implantable housing 36which contains a microcontroller/CPU 100, read only memory (ROM) 102,sensing hardware 104, including atrial and ventricular sense amplifiers(not shown); therapy delivery hardware 106, and telemetry hardware 108.All electronic components of the pulse generator 22 are interconnectedby way of a bus connection 101. The method of the system 20 isimplemented in an algorithm as firmware within the ROM 102 and isexecuted by the microcontroller/CPU 100. The sensing hardware 104 isalso connected to the microcontroller/CPU 100, and contains a pluralityof electrical connections 110 coupled to the atrial and ventricularsense amplifiers. The output of the sense amplifiers is connected to themicrocontroller/CPU 100, so that the atrial 32 and the ventricle 34cardiac signals received through the sensing hardware 104 are analyzedby the algorithm within the microcontroller/CPU 100.

The microcontroller/CPU 100 is also coupled to the therapy deliveryhardware 106, which controls the delivery of electrical energy to theheart 30 through a plurality of electrical output connections 112 toaffect the sinus rhythm of the heart 30 under certain combinations ofatrial 32 and ventricular 34 conditions. Power to the implantable pulsegenerator 22 is supplied by an electrochemical battery 114 that ishoused within the implantable pulse generator 22. The implantable pulsegenerator 22 is interrogated and programmed via bidirectional radiofrequency telemetry through the telemetry hardware 108 with an externalprogrammer.

Referring again to FIG. 1, a connector block 38 is mounted on theimplantable pulse generator 22. The connector block 38 has two connectorports to physically and electrically connect the atrial catheter 24 andthe ventricular catheter 26 to the sensing hardware 104 and the therapydelivery hardware 106 of the implantable pulse generator 22. Additionalconnector ports can be added to the connector block 38, andconfigurations with three or more ports are known. Alternatively, theconnector block can be provided with one connector port for physicallyand electrically connecting an implantable transvenous catheter to theimplantable pulse generator 22.

The electrical activity in the heart 30 is sensed and therapies aredelivered to the heart 30 through at least one transvenouspacing/defibrillation lead connected to the implantable pulse generator22. Unipolar and/or bipolar pacing and sensing electrodes can be used inconjunction with the at least one transvenous pacing/defibrillationlead. In the embodiment shown in FIG. 1, bipolar leads and sensingcircuits are utilized for sensing both the atrial 32 and the ventricular34 activity. Sensing atrial activity includes the determination ofatrial P-waves for the purpose of determining atrial intervals, andventricular activity is monitored by sensing for the occurrence ofventricular R-waves for the purpose of determining ventricularintervals. Pacing therapies to the atrium 32 or ventricle 34 aredelivered to the heart 30 using these same leads. The system 20 also hasdefibrillation electrodes which are connected to the electrical outputconnections 112, and serve to deliver cardiovertion and defibrillationlevel electrical pulses to the heart 30 upon a signal from themicrocontroller/CPU 100 indicating a predetermined condition within theheart 30. The housing 36 of the system 20 is an optional defibrillationelectrode, where the housing 36 of the implantable pulse generator 22 iselectrically connected to a cathode pole of the therapy deliveryhardware 106. All defibrillation electrical pulses are delivered to theheart with at least two defibrillation electrodes, or through at leastone defibrillation electrode and the housing 36 of the implantable pulsegenerator 22. The system 20 supports a plurality of pacing regimens,including DDD pacing.

Besides the lead configuration shown in FIG. 1, the system 20 supportsseveral other lead configurations and types. For example it is possibleto use ventricular epicardial rate sensing, atrial endocardial bipolarpace/sensing, ventricular endocardial bipolar pace/sensing, epicardialpatches, and Endotak® Series and ancillary leads in conjunction with theimplantable pulse generator 22.

Referring now to FIG. 1, there is shown an embodiment of the system 20of present invention where the atrial catheter 24 has an elongate body40 having a peripheral surface 42, proximal and distal ends, 44 and 46,a first atrial electrode 48 and a second atrial electrode 50 on theperipheral surface 42. The first atrial electrode 48 and the secondatrial electrode 50 receive bipolar electrical cardiac signals from theright atrium chamber 52 of the heart 30, and are attached on theperipheral surface 42 of the elongate body 40.

The first atrial electrode 48 is at or adjacent to the distal end 46 ofthe elongate body 40 and is either a pacing tip electrode or asemi-annular electrode partially encircling or an annular electrodeencircling the peripheral surface 42 of the elongate body 40. The secondelectrode 50 is an annular electrode encircling or semi-annularelectrode partially encircling the peripheral surface 42 of the elongatebody 40. The second electrode 50 is spaced longitudinally along theperipheral surface 40 from the first atrial electrode 48 and the distalend 46 of the atrial catheter 24 such that when the atrial catheter 24is inserted into the right atrial chamber 52 of the heart 30, the firstatrial electrode 48 is in physical contact with a portion of a wall ofthe right atrial chamber 52 of the heart 30 and the second electrode 50is within the right atrium chamber 52.

Electrical leads extend longitudinally within the elongate body 40 ofthe atrial catheter 24 from a connection end at the proximal end 44 andmake connection to the first and second atrial electrodes 48 and 50. Theproximal end 44 of the atrial pacing catheter 24 is releasably attachedto the connector block 38 of the implantable pulse generator 22 with thecontact ends of the electrical leads in electrical connection with boththe sense amplifiers of the sensing hardware 104 and the therapydelivery hardware 106 such that the implantable pulse generator receivesbipolar signals from and delivers bipolar pacing to the right atrium 52of the heart 30.

The ventricular catheter 26 comprises an elongate body 54 having aperipheral surface 56, proximal and distal ends, 58 and 60, and aventricle pacing electrode 62, a first defibrillation electrode 64, anda second defibrillation electrode 66 on the peripheral surface 56 of theelongate body 54. The ventricular pacing electrode 62 and the firstdefibrillation electrode 64 are adapted to receive electrical cardiacsignals from the right ventricle chamber 68 of the heart 30, and areattached on the peripheral surface of the elongate body 54. The seconddefibrillation electrode 66 is space apart and space longitudinally onthe peripheral surface 56 of the ventricular catheter 26 to affordpositioning the ventricular catheter 26 in the heart 30 with theventricular pacing electrode 62 in the apex of the right ventricle 68,the first defibrillation electrode 64 within the right ventricle chamberof the heart, and the second defibrillation electrode 66 within theright atrium chamber 52 or a major vein leading to right atrium.

Electrical leads extend longitudinally within the elongate body 54 ofthe ventricular catheter 26 from a connection end at the proximal end 58to make connection with the ventricular pacing electrode 62, the firstdefibrillation electrode 64, and the second defibrillation electrode 66.The proximal end 58 of the ventricular catheter 26 is releasablyattached to the connector block 38 of the implantable pulse generator 22with the contact ends of the electrical leads in electrical connectionwith both the sense amplifiers of the sensing hardware 104 and thetherapy delivery hardware 106 such that the implantable pulse generator22 receives either unipolar or bipolar signals from and can deliverunipolar or bipolar pacing to the right ventricle 68 and defibrillationelectrical pulses to the ventricles 34 of the heart 30.

The atrial catheter 24 and the ventricular catheter 26 are releasablyattached to and are separated from the implantable pulse generator 22 tofacilitate inserting the atrial catheter 24 into the heart 30. Theatrial and ventricular catheters, 24 and 26, are inserted into the heart30 transvenously through a cephalic or subclavian vein to position thedistal end 46 of the atrial catheter 24 in the right atrium chamber 52and the distal end 60 of the ventricular catheter 26 in the apex of theright ventricular chamber 68. The proximal end 44 of the atrial catheter24 and the proximal end of the ventricle catheter 26 are then attachedto the implantable pulse generator 22. The proximal end 44 of the atrialcatheter 24 and the proximal end 58 of the ventricular catheter 26 areadapted to seal together with the connector ports of the implantablepulse generator 22 to thereby engage the contact ends of the atrialcatheter 24 and the ventricular catheter 26 with the plurality ofelectrical connections 1110 and the therapy delivery hardware 106 of theimplantable pulse generator 22. The implantable pulse generator 22 ofthe system 20 is then positioned subcutaneously within the body 26.

Referring now to FIG. 3, there is shown a flow diagram of a firstportion of the manner in which the electronic circuitry of the system 20operate according to the present invention. Detection decisions by thesystem 20 are based on detected cardiac cycle lengths. Initially, thesystem 20 monitors and analyzes sensed ventricular events within theheart 30 during a ventricular monitoring step 300. During theventricular monitoring step 300, the system senses the occurrence ofventricular R-waves, from which a ventricular interval rate 302 (i.e.,an R-R wave time interval) is calculated. The system 20 then calculatesan average ventricular rate from the sensed ventricular interval rate304.

The system 20 can be programmed to define one, two, or three rate zonesthat are above a defined upper limit for normal ventricular intervalrates. A rate zone is a range of ventricular interval rates which isdefined between a lower rate threshold, and a lower rate threshold ofthe next faster rate zone (if any) programmed in the system 20. For eachrate zone, the lower rate threshold is a programmable value in beats perminute (bpm) and is the value to which the system 20 compares eachsensed ventricular interval rate to determine the zone in which thatventricular interval rate belongs. Rate zones can be defined for slowtachycardia (VT-1), fast ventricular tachycardia (VT), and ventricularfibrillation (VF). When only one rate zone is programmed, the systemonly monitors for the VF rate zone; when two rate zones are programmedthe system monitors the VF and the VT rate zones; and when three ratezones are programmed the system monitors the VF, VT, and the VT-1 ratezones.

The lower rate threshold for a one-zone configuration, (i.e., only theVF rate zone is programmed) can be a programmable value between 130 to250 bpm. For a two- or three-zone configuration the lower rate thresholdfor the VT rate zone can be programmable between 110-210 bpm and for theVT-1 rate zone the lower rate threshold can be programmable between90-200 bpm.

To determine if an individual ventricular interval rate falls into aprogrammed rate zone, the system 20 detects the intervals between aseries (e.g., 4-16, 6-14, or 8-12) of the most recent consecutiveventricular R-waves. Detecting the interval between 10 of the mostrecent consecutive ventricular R-waves is considered to be a goodsampling. This sampling is called a detection window, with a newdetection window occurring with each consecutive ventricular R-wave. Thesystem assesses the ventricular interval in relation to one of the ratezones. The use of detection windows helps to differentiate and classifyventricular tachyarrhythmias into a rate zone, and helps to ensure thatthe correct ventricular therapy is delivered to the patient.

As each new ventricular interval rate is measured in the ventricularmonitoring step 300, it is compared to each rate zone's lower ratethreshold in the detection window analysis step 306. Based on thiscomparison, the ventricular interval rate is classified 308 as beingeither a fast or a slow ventricular interval with respect to each of therate zones. A slow ventricular interval for a given rate zone has aventricular interval rate that is less than that rate zone's lower ratethreshold, and a fast ventricular interval for a given rate zone has aventricular interval rate that is equal to or greater than the ratezone's lower rate threshold. When a predetermined percentage of theventricular interval rates within the detection window are classified asbeing fast ventricular intervals for a rate zone 310 (i.e., thatpredetermined percentage can be a value between 65-95, 70-90, or 75-85percent of the ventricular interval rates, with 80 percent being anacceptable value) the system 20 is “satisfied” that the ventricular rateintervals for the heart are properly classified in that rate zone.Herein this condition is referred to as the detection window beingsatisfied 310. If the detection window is not satisfied, the system 20returns to the ventricular monitoring step 300.

When a rate zone detection window becomes satisfied that the ventricularrate interval for the heart is properly classified in one of the VF, VT,or VT-1 rate zones, the system 20 switches to a cardiac episodecondition and starts a duration time interval 312. A different durationtime interval is associated with each rate zone. The duration timeinterval times a length of time during which the system 20 continues tomonitor and analyze the ventricular interval rates within the detectionwindows to ensure that the ventricular tachyarrhythmia of a satisfiedrate zone is sustained, rather than transitory, before the system 20initiates a selected treatment. The length of each duration timeinterval is programmed for each rate zone. For example, the durationtime interval for the VF rate zone can be programmed for a durationbetween 1-15 seconds, the duration time interval for the VT rate zonecan be programmed for a duration between 1-30 seconds, and the durationtime interval for the VT-1 rate zone can be programmed for a duration ofbetween 1-60 seconds. The duration time programmed for any lower ratezones must be equal to or greater then the duration time interval for ahigher rate zone.

Once the system 20 becomes “satisfied” that the ventricular rateinterval for the heart is properly classified in that rate zone, thesystem 20 also starts a rate zone remaining satisfied step 314, wherethe system 20 continues to monitor the ventricular interval rates withinthe shifting detection window to be sure that the ventricular rateinterval for the heart remains properly classified in that rate zone.

This condition is satisfied as long as at least a maintenance percentageof the ventricular interval rates in the subsequent detection windowsremain classified as fast ventricular intervals within the satisfiedrate zone. The maintenance percentage of the ventricular interval set isa value between 45-75, 55-65, or 55-65 percent of the ventricularinterval set, where 60 percent is a acceptable value. Herein thiscondition is referred to as the detection window remaining satisfied.

The duration time interval continues to elapse so long as a satisfieddetection window remains satisfied. The duration time interval ischecked by the system 20 during each ventricular interval to determineif the duration time interval has expired 314. If at any point thesystem 20 detects fewer than the maintenance percentage of fastventricular intervals, the duration time interval for that rate zone isreset to zero, and the system 20 returns to the ventricular monitoringstep 300 for that particular rate zone which failed to remain satisfied.

The duration time interval for a rate zone mms independently of anyother rate zone duration time interval. A hierarchy of rate zoneduration times exists where a higher rate zone duration time intervaltakes precedence over a lower rate zone timer. Thus, if both a higherand a lower rate zone become satisfied, the lower rate zone durationtime interval will continue to elapse, but will be ignored while thehigher rate zones' duration time interval is elapsing. If the higherrate zone's duration time interval expires and the rate zone hasremained satisfied, therapy for that higher rate zone will be initiatedregardless of the state of the lower rate zone timer. If, however, thehigher rate zone's detection window does not remain satisfied, then theduration time interval for the lower rate zone is no longer ignored andtherapy for the lower rate zone will be initiated when its duration timeinterval expires, provided the lower rate zone remains satisfied and nohigher rate zone window becomes satisfied. Alternatively, if a detectionwindow is satisfied for a lower rate zone, subsequent ventricularinterval rates that are classified in a higher rate zone, or rate zones,would be counted for keeping the lower rate zone satisfied and forsatisfying the higher rate zone. If the higher rate zone were to becomesatisfied prior to the satisfied lower rate zones duration time intervalexpiring, the system 20 would deliver therapy only at the expiration ofthe higher rate zone duration time interval.

When the highest satisfied rate zone duration time interval expires andthe last detected ventricular interval rate falls within that rate zone,therapy is initiated. This function is carried out in a last beat inzone detection step 316, where the system 20 ensures that the propertherapy is selected in the presence of a changing ventricular rate. Ifthe last detected interval does not remain in the highest satisfied ratezone, the system 20 forces a dely in initiating therapy until the nextventricular interval by returning to the rate zone remaining satisfiedstep 312. Each subsequent interval will be analyzed until theventricular interval rate is within the highest satisfied rate zone, orthe window for the highest satisfied rate zone fails to remainsatisfied. If the next interval is within the highest satisfied ratezone, therapy will be initiated 318. If it is not within the highestsatisfied rate zone, the next interval is examined, and the process isrepeated.

During any such delay because the next interval is not within thehighest satisfied rate zone the system 20 can fail to remain satisfiedif the ventricular interval rate decreases and the rate zone drops belowthe maintenance percentage. Also, during the delay there is thepotential for a higher rate zone window to become satisfied if theventricular interval rate were to increase. If a detection window for ahigher rate zone does become satisfied, then the therapy selection isdelayed until either the higher rate zone duration time interval expiresor the detection window for the higher zone no longer remains satisfied.This ensures that the system 20 will provide the appropriate therapy forthe highest satisfied rate zone. On the other hand, if for a satisfiedrate zone, the number of ventricular intervals within or above that ratezone falls below the maintenance percentage the system 20 will thenreturn to the ventricular monitoring step 300.

Once the last beat in zone criteria is met, the system 20 proceeds toinitiate and deliver a ventricular tachycardia therapy in an initiatetherapy step 318. The system 20 can be programmed to deliver two typesof therapy to terminate ventricular tachycardia or fibrillation:antitachycardia pacing (ATP) and/or cardioversion/defibrillation shocks.Antitachycardia pacing schemes consist of bursts of pacing pulses thatare delivered between the ventricular rate-sensing electrodes. Shocksare high-voltage truncated exponential pulses (monophasic or biphasic)delivered through the shocking leads synchronously with detected heartactivity.

A ventricular tachycardia therapy is a therapy regimen delivered in aparticular rate zone, and can consist of a combination of ATP andcardioverting/defibrillating shocks. Each rate zone can be independentlyprogrammed with a ventricular tachycardia therapy. AntitachycardiaPacing (ATP) is used to deliver a series of pacing pulses to theventricle in order to interrupt pace-terminable ventricular tachycardiaATP ventricular tachycardia therapies can be programmed as off, burst,ramp, scan, or ramp/scan in the VT and the VT-1 rate zones. For ratezones VT-1 and VT, a maximum of two ATP therapies and fivecardioverting/defibrillating shocks are available to be programmed intothe system 20. ATP therapy is not available in the VF rate zone of anyconfiguration.

Five cardioverting/defibrillation shocks are programmable into thesystem 20. The shock strength of the first and second cardioverting ordefibrillation shocks delivered in each rate zone are programmablevalues between 0.1-29 joules. The last three potentialcardioverting/defibrillation shocks are given at the maximum shockoutput of 29 joules. All shocks are delivered synchronously with asensed ventricular cardia event (R-wave) if possible to avoidaccelerating a tachycardia into fibrillation. The shock therapy consistsof programmable monophasic or biphasic cardioversion/defibrillationshocks. In addition, the polarity of the shocks is programmable, where aduty cycle of 60/40% for the biphasic waveform is considered to be anappropriate value. Once selected, the waveform type and polarity applyto all shocks delivered by the apparatus.

When programming therapies, the therapies within a rate zone must beordered in ascending therapy strengths. All ATP therapies are consideredto be of equal strength, but are of lower strength than any shocktherapy. Whenever a shock therapy has been delivered, no further ATPtherapy is allowed, since ATP therapy is of lower strength than shocktherapy. The strength of the shock therapies is determined from theprogrammed energy. In a multiple rate zone configuration, therapies in ahigher rate zone can be of lesser strength or equal to those in a lowerrate zone; however, within each zone the therapies must be programmed inequal or increasing energy output.

In a VT-1 rate zone of a three-zone configuration, some or all of theshocks may be programmed off, starting with the maximum shocks first. Ifthe maximum shocks are programmed off, then the second shock can beprogrammed off, and if the second shock is programmed off, then thefirst shock can be programmed off. If the arrhythmia persists in theVT-1 rate zone when some or all of the shocks are programmed off, nofurther therapy will be delivered unless the arrhythmia accelerates to ahigher rate zone.

Based on initial detection criteria, the system 20 selects the firstprescribed therapy in the rate zone in which the tachyarrhythmia isdetected. After delivering the selected therapy, the system 20 beginsredetection to determine whether the arrhythmia has been converted. Ifthe arrhythmia is converted to a rate below the lowest programmedthreshold, the system 20 continues monitoring until the cardiac episodeends. When the cardiac episode ends, the system 20 will again useinitial detection criteria for a new cardiac episode. When a new cardiacepisode is declared, the first prescribed therapy will be deliveredagain. If the arrhythmia is not converted and an arrhythmia isredetected in the same zone, the next programmed therapy in that zone isselected and delivered, followed again by redetection. If the arrhythmiapersists in the same rate zone the therapy will progress in that ratezone. If an arrhythmia crosses a rate zone (accelerates or decelerates)following therapy delivery and is redetected in a higher or lower ratezone, a therapy of equal or greater strength than the previouslydelivered therapy is selected form the detected zone and delivered. Thesystem 20 determines which shock to deliver prior to capacitor charging,based on the detected rate threshold. If during capacitor charging, thetachyarrhythmia accelerates or decelerates from the initially detectedrate, the predetermined energy will be delivered. Redetection isperformed after each therapy delivery to determine if further therapy isrequired.

After therapy delivery, the system 20 employs a redetection criteria toevaluate the rhythm and determine whether more therapy is appropriate.When redetection criteria are satisfied, the rules for therapy selectionthen determine the type of therapy to deliver. If an ATP scheme is beingdelivered, the system 20 monitors the cardiac rate after each burst andemploys the same type of detection window described above, looking foreighty percent of the intervals to be fast intervals, and theredetection duration time interval to determine if the tachyarrhythmiahas terminated. The ATP scheme will continue with the next burst in thesequence until any one of the following conditions is satisfied:Redetection has declared that therapy is successful (end-of-episode),the specified number of ATP bursts in the scheme has been delivered, theATP time-period for the zone has expired, the detected tachyarrhythmiarate changes to a different rate zone whereby a different therapy isselected, stability analysis forces therapy to skip over ATP therapy toinitiate shock therapy.

If shock therapy is being delivered, the system 20 monitors the cardiacrate after each shock and employs the same type of detection windowdescribed above, looking for eighty percent of the interval to be fastintervals, and the post-shock duration time to determine if thetachyarrhythmia has terminated. Shock therapy will continue until one ofthe following conditions is satisfied: redetection has declared thattherapy is successful (end-of-cardiac episode), all five availableshocks have been delivered for a cardiac episode, all shocks programmedin the VT-1 zone have been delivered and the rate stays in the VT-1zone. If all available shock have been delivered for a cardiac episode,no further therapy is available until the system 20 monitors a ratebelow the lowest rate threshold for 30 seconds and end-of-cardiacepisode is declared.

Cardiac episodes can be classified as either treated or non-treatedwhere a treated episode is one in which a satisfied detection windowremains satisfied and the therapy is delivered. A non-treated episode isone in which a detection window is initially satisfied but therapy isnot delivered because the detection window fails to remain satisfiedduring the term of the duration time interval. A cardiac episode isdeclared complete when either the system 20 has terminate the cardiacepisode by delivering therapy or when the cardiac episode has terminatednaturally. For a treated episode, an end of episode timer starts at thepoint that therapy is delivered. For a non-treated episode, anend-of-episode timer starts at the point that the system 20 determinesthat all detection windows are no longer satisfied. The end-of-cardiacepisode time intervals is intended to allow the patient to stabilizebefore initial detection and initial therapy are used again. The episodewill be declared complete if the detection windows do not becomesatisfied for a specified time following the last delivered therapy. Fora non-treated (no therapy delivered) the elapsed time required todeclare episode over (end-of-episode timer) is 10 seconds, for treatedwhere only ATP therapy delivered it is 10 seconds, and treated where anyshock therapy delivered it is 30 seconds. If any window becomesresatisfied prior to the episode time-period being reached, theend-of-cardiac episode timer is rest to zero. It will start again wheneither therapy is delivered or all windows are not satisfied. Once acardiac episode has been declared complete, the system 20 will applyinitial detection and therapy to subsequent tachyarrhythmias. When thecardiac episode is terminated naturally, the cardiac episode is completewhen all detection windows are no longer satisfied and remainunsatisfied for the duration of an end-of-episode time-out. Theend-of-episode time-out timer is started when all detection windows areno longer satisfied. An end-of-episode time-out is intended to allow thepatient's heart to stabilize before allowing the programmed therapysequence to restart at the beginning of the algorithm.

If a rate zone detection window becomes satisfied before anend-of-episode time-out is reached, the rate duration time intervalassociated with that window is reset and the rate zone therapy in theprogrammed sequence is delivered once the rate duration time intervalexpires. If the end-of-episode time-out is reached and a rate zonedetection window has not been satisfied, the cardiac episode isterminated. When this occurs, the current rate zone therapy sequence isterminated and the detection windows are cleared. If any detectionwindow subsequently becomes satisfied, a new cardiac episode begins, andthe analysis process starts again.

At the start of a cardiac episode the system 20 senses the atrialinterval rate in addition to the ventricular rate and analyzes thecardia rhythms (both ventricular and atrial) to assess (1) whether theventricular interval rate is or is not greater than the atrial intervalrate by at least a bias factor (herein referred to as “V>A+B”), (2) thepresence of atrial fibrillation, (3) the onset rate of the ventriculararrhythmia (herein referred to as “Onset rate”), and (4) the stabilityof the ventricular arrhythmia (herein referred to as “Stability”). Theseassessments are made by a series of programmable detection enhancementalgorithms, which provide information about the nature of theventricular tachycardia and the physiological state of the atria duringa detected ventricular arrhythmia tachycardia.

The system 20 can be programmed so that under predeterminedcircumstances the detection enhancement algorithms are therapyinhibitors, inhibitor overrides, or therapy accelerators of theventricular tachycardia and ventricular fibrillation therapy. Detectionenhancements are not available for use with the VF rate zone becauseventricular fibrillation requires immediate treatment to save thepatient's life. For a two-zone configuration, certain of the detectionenhancements can be used with the VT rate zone. These include thedetection enhancements of V>A+B, the presence of atrial fibrillation,Onset rate, and Stability (any of which can be used as an inhibitor(explained below) or as an accelerator (explained below); although,Stability analysis cannot be programmed as both an inhibitor and anaccelerator in the same rate zone, but can be programmed as an inhibitorwhen other detection enhancements are programmed as inhibitors andprogrammed as an accelerator only when no other detection enhancementsare programmed as inhibitors). Also included is the use of a sustainedrate duration time-period, which will also be discussed below. For athree zone configuration the detection enhancement of Stability as anaccelerator can be programmed for use with the VT rate zone, and thedetection enhancements of V>A+B, the presence of atrial fibrillation,Onset rate, and Stability used as an inhibitor to delivering therapy canbe programmed for use with the VT-1 rate zone. The sustained rateduration time-period can also be used with the VT-1 rate zoneprogramming.

A detection enhancement programmed as a therapy inhibitor can cause thedelivery of a satisfied rate zone therapy to be delayed or inhibited ifcertain enhancement criteria are not satisfied at the end of theduration time interval. For example, the Onset rate detectionenhancement can be programmed to inhibit therapy if the patient's heartrate increases gradually, as could occur when the patient begins toexercise. Also, the Stability detection enhancement can be programmed toinhibit therapy delivery if the ventricular rhythm is unstable so thatthe system 20 will inhibit delivering ventricular therapy when theunstable ventricular rhythm has its origins in the atria, because inorder for an ATP therapy to be effective, the system 20 must anticipatethe occurrence of the next beat so as to break into the re-entrant loopof the ventricular tachycardia This may not be possible if theventricular interval varies widely from beat to beat. The atrialfibrillation threshold and corresponding ventricular tachycardiastability enhancements can be programmed to inhibit ventricular therapyif the atrial rhythm is fast and the ventricular rhythm is unstable.

A detection enhancement therapy inhibitor may, however, be overridden bythe use of a sustained rate duration (SRD) time-period. The SRDtime-period is a time interval that is started at the expiration of aduration time interval for a satisfied rate zone that is the lowest ratezone in a multiple rate zone configuration. The SRD time-period timercan be used in conjunction with detection enhancements and it enablesthe system 20 to override the detection enhancement therapy inhibitors(atrial fibrillation rate threshold, Onset rate, and/or Stabilityprogrammed to inhibit if the ventricular rate is unstable) and deliverthe therapy for the satisfied rate zone if the ventricular tachycardiais sustained for the programmed SRD time-period. The SRD time-period isnot used in conjunction with the Stability detection enhancementprogrammed as an accelerator.

As previously mentioned, the SRD time-period is programmed only in thelowest rate zone of a multiple rate zone configuration (e.g., VT for atwo-zone configuration, and VT-1 for a three rate-zone configuration),and is programmed within a range of values of from 10 seconds to 60minutes, or to be off. The SRD time-period is used only when a detectionenhancement programmed as a inhibitor is programmed on. If a detectionenhancement as an inhibitor is inhibiting therapy and the detectionwindow is remaining satisfied, the SRD time-period begins at the end ofthe duration time interval. If the detection window continues to remainsatisfied during the SRD time-period, the programmed therapy will bedelivered at the end of the SRD time-period. If the rate accelerates toa higher rate zone and the duration time interval for the higher ratezone is satisfied, therapy is initiated without waiting for SRD timeperiod to time out.

Detection enhancement can also be programmed as an inhibitor override tocause one or more therapy inhibitors to be bypassed and therapydelivered if certain criteria are satisfied. For example, the V>A+Bdetection enhancement can be used to override the therapy inhibitorsdescribed above if the system 20 determines that the ventricular rate isgreater than the atrial rate plus a bias factor.

Detection enhancements can be programmed as therapy accelerators tocause an acceleration of a ventricular therapy sequence whereby thesystem 20 skips over any remaining rate zone prescription therapies inthe programmed sequence for that zone, and delivers the first shocktherapy. Therapy accelerators accelerate the sequence of therapy byskipping over or interrupting an ATP scheme to initiate charging for thefirst programmed shock (which may be low or high energy) for the ratezone. The stability detection enhancement can be programmed toaccelerate delivery of shock therapy if the system 20 determines therhythm of the ventricle is unstable.

Referring to FIG. 4 there is shown a flow diagram of the detectionenhancements which can be programmed and enabled within the system 20.The V>A+B, Onset rate, and Stability detection enhancements are analyzedsimultaneously (if at all) following a detection window becomingsatisfied. The AFib detection enhancement is analyzed following theexpiration of a duration time interval for a satisfied rate zone.Therefore, FIG. 4 should not be taken to indicate any order in theanalysis of atrial and/or ventricular information. Also shown in FIG. 4are three different possible modifications to the therapy delivered tothe heart based upon the results of the detection enhancements. Thesepossible modifications to the therapy to be delivered include the resultnumbered number 1, which inhibits delivering therapy until the SRDtime-period has expired; the result numbered number 2, which does notinhibit the delivery of therapy; and the result numbered number 3, whichinhibits delivering therapy until the SRD time-period has expired oruntil the ventricular rhythm becomes stable as assessed by the Stabilitydetection enhancement.

When the system 20 senses a cardiac episode (i.e., a detection windowbecome satisfied), the system 20 has already analyzed and calculated theventricular and the atrial interval rates in the ventricular monitoringstep 300. If a V>A+B detection enhancement 402 is enabled in section 400of FIG. 4, and a detected ventricular tachycardia is dissociated fromthe atrium (i.e., the ventricular tachycardia is dissociated from theatrium if the ventricular rate is greater than the atrial rate by atleast a bias factor), the system 20 can make use of atrial cycle lengthinformation to bypass additional programmed detection enhancements(e.g., to bypass Onset rate and/or Stability and/or Atrial fibrillationas inhibitors) and initiate therapy for the satisfied rate zone, as isindicated by the “Treat” symbol 404.

To determine if the ventricular rate is greater than the atrial rate byat least the bias factor, the section 400 of the system 20 determinesthe sum of a ventricle set of detected ventricular cycle lengths (e.g.,last 10) and the sum of a atrial set atrial cycle lengths (e.g., last10) prior to the end of the duration time interval. The last 10 atrialintervals prior to the end of the duration time interval are assessedafter a third fast ventricular interval is detected by the system 20. Iffewer than 10 atrial intervals are available, then the intervalsavailable will be used to calculate the average atrial rate. The cyclelength sums are converted to ventricular rate averages and atrial rateaverages by the section 400. If the ventricular rate average is greaterthan the atrial rate average by the bias factor then the ventricle isdetermined to be beating faster than the atrium and therapy is initiatedbased on the highest satisfied ventricular rate zone. All otherdetection enhancements that are active are bypassed if the V>A+Bdetection enhancement is met. If the ventricular rate is not greater,then therapy is inhibited. The value of the bias factor, B, isprogrammable in the ranges of 5-20 bpm, where 10 bpm is an appropriatenumber.

Atrial information can also used in the atrial fibrillation detectionenhancement 406 for assessing whether the atria are in a state of atrialfibrillation. In fibrillation detection enhancement 406, the system 20monitors the atrial interval rate and compares it to a preprogrammedatrial fibrillation (AFib) rate threshold value which is used todetermine the existence of an atrial fibrillation. Therapy to theventricles is withheld if the atrial interval rate is above the AFibrate threshold value and the conduction of the ventricular indicatesthat the underlying cause of a high ventricular rate is due to aventricular response to fibrillation in the atrium.

The atria are determined to be in fibrillation in the following manner.When the lowest satisfied rate zone's rate duration time interval hasexpired, a first set of atrial intervals, (e.g., the 10 most recentatrial intervals prior to the expiration of the duration time interval)are examined. Each atrial interval is classified as being either shorterthan or longer than the programmed atrial fibrillation rate thresholdvalue AFib. If a predetermined majority number (6 of the last 10 atrialintervals) of the first set of atrial intervals are shorter than theatrial fibrillation interval rate threshold value, the atrial rhythm isdeclared to be atrial fibrillation. Ventricular stability is thenchecked (i.e., when the atrial fibrillation detection enhancement isprogrammed on, the Stability detection enhancement is also activated)and if the system 20 determines that the ventricles are unstable,ventricular therapy will be withheld. In the event that therapy is notdelivered, the atrial rate will continue to be examined by the system 20and as long as a predetermined quorum number (e.g., 4 of 10 subsequentsets of atrial intervals) of subsequent sets of atrial intervals remainshorter than the atrial fibrillation interval rate threshold value,atrial fibrillation is determined to be continuing. Atrial fibrillationrate threshold values can be programmed within the ranges of about 250to 400 bpm, with 250 bpm being a useful value. The atrial fibrillationdetection enhancement can also be programmed off.

The Onset rate detection enhancement 408 measures the rate of transitionof a ventricular interval rate from a slower sinus rate to tachycardiarate. The Onset rate detection enhancement 408 is intended to enable thesystem 20 to differentiate physiologic sinus tachycardias, whichtypically begin slowly and have a gradual onset, from pathologicaltachycardias, which typically begin in a more abrupt step-like manner.The programmable Onset rate detection enhancement at 408 is limited tothe lowest zone of multiple rate zone configuration (e.g., VT for a tworate zone configuration and VT-1 for a three rate zone configuration)and may be used in conjunction with the SRD time-period.

Referring now to FIG. 5, there is shown a schematic of a series ofventricular intervals used to assess the Onset rate detectionenhancement. When a detection window becomes satisfied, the system 20assesses the Onset rate enhancement by comparing a cycle length changeof a pair of adjacent ventricular intervals with a programmed onset ratevalue in a two step procedure. In the first step, the system 20 locatesa pair of adjacent ventricular intervals 500 where the cycle lengthbetween the ventricular intervals has decreased the most This pair ofventricular intervals 500 is called a pivot point, and it is determinedusing ventricular intervals sensed prior to the start of a cardiacepisode 510.

To calculate the pivot point 500, the system 20 begins with the intervalthat initially satisfied the detection window and started the cardiacepisode 510. This interval is called interval −0, and the system 20scans up to 26 previous ventricular intervals looking for the pivotpoint. Interval differences are calculated as(V−V)_(interval(n))−(V−V)_((interval(N-1)), where n=−3 to −25. Pivotpoint scanning begins at n=−3, because three intervals are requiredafter a potential pivot point interval for the stage two evaluation.Pivot point scanning ends at n=−25 because six intervals are requiredbefore the pivot point for the baseline average ventricular rate valuecalculations.

A programmed onset threshold value is used in assessing the pivot point500. The system 20 compares the pivot point to the programmed onsetthreshold value to determine if the pivot point has exceeded thethreshold value. If the pivot point decrease has exceeded the programmedthreshold value, the system 20 compares the ventricular intervals beforeand after the pivot point to ensure that the overall ventricular ratehas changed by more than the programmed threshold value. The overallventricular rate change and the programmed threshold value are comparedby the system 20 in either absolute time or as a percentage ofventricular interval change. If the pivot point decrease is greater thanor equal to the programmed threshold value, then the first stage testresult is considered sudden. The programmable ranges for the percentageanalysis are between 9 to 50% and for the absolute time analysis arebetween 50 to 250 milliseconds. The Onset rate enhancement can also beprogrammed off. The selected Onset rate value represents the minimumdifference that must exist between ventricular interval rates that arebelow the lowest programmed rate threshold and the ventricular intervalsthat are above the lowest programmed rate threshold.

For a programmed onset threshold in absolute time, the system 20compares the pivot point decrease to the onset threshold value usingabsolute time values. If the decrease is greater than the onsetthreshold value, stage one is satisfied. For threshold as a percentageof interval, the comparison is made using four of six intervals prior tothe pivot point to calculate a baseline average ventricular rate value,where the Baselineaverage=[(V−V)_(pp(−3))+(V−V)_(pp(4))+(V−V)_(pp(−5))+(V−V)_(pp(−6))]/4,where the values are seen at 520 on FIG. 5. The first two ventricularintervals prior to the pivot point 530 are skipped to avoid countingpremature ventricular contractions during a rapid sinus tachycardia. Thebaseline average is then multiplied by the onset threshold as a percentto convert the threshold into the units of milliseconds (ms). If thepivot point interval decrease is greater than the onset threshold as apercent of interval in ms, then stage one is satisfied, and if not,stage one is not satisfied.

If stage one is satisfied, the system 20 proceeds to stage two. In stagetwo, the baseline average 520 is compared to the value of the pivotpoint interval and each of the subsequent three ventricular intervals.If the difference between the baseline average and at least three of thefour intervals (i.e., the pivot point interval and the three ventricularintervals) is greater than or equal to the onset threshold value, thenthe onset is declared sudden. If fewer than three intervals are greaterthan the onset threshold value, then the onset is declared gradual. Thistest ensures that the ventricular rate has changed by more than theprogrammed threshold value over a wider set of ventricular intervalsamples.

If either stage indicates a gradual onset, therapy will be inhibited inthe lowest zone. Therapy will be delivered only if the rate acceleratesto a higher zone, the SRD time-period timer expires, or information fromthe atrial lead determines that the V>A+B detection enhancement has beensatisfied.

Referring again to FIG. 4, the Stability detection enhancement 410 isused to distinguish variable ventricular interval rates from stableventricular interval rates. The system 20 uses the Stability detectionenhancement 410 to determine the stability of a ventriculartachyarrhythmia using a weighted average variance of a series of sensedventricular intervals. This degree of variability, when used by itself,may allow the system 20 to distinguish atrial fibrillation (which mayproduce greater R-R variability) from monomorphic ventriculartachycardia (MVT) (which is typically stable and pace terminable), andalso may help differentiate MVTs from ventricular fibrillation andpolymorphic ventricular tachycardias (which are not typically to paceterminable). Based on this differentiation, the system 20 can beprogrammed to deliver therapy by programming the stability detectionenhancement to inhibit delivering therapy at the termination of theduration time interval if the ventricular rate is determined to beunstable. Alternatively, the system 20 can deliver therapy when, at theend of the duration time interval, the ventricular rate is determined tobe stable. Assessment of ventricular rate stability begins when theduration time interval starts in a satisfied rate zone.

Referring now to FIG. 6, the stability detection enhancement usesventricular interval differences to assess the stability of theventricular rhythm. Both the ventricular interval differences and anaverage difference are calculated while the duration time interval iselapsing. The variance from ventricular interval to ventricular interval(VAR(n)) 600 is determined by subtracting each current ventricularinterval from the previous ventricular interval using one second as thegreatest difference where VAR(n)=absolute value (RR(n−1)−RR(n)). TheStability detection enhancement is “seeded” (VAR_(SEED)) by firstdetermining the average variance of the four interval pairs immediatelybefore the start of duration time interval 610, whereVAR_(SEED)={VAR(1)+VAR(2)+VAR(3)+VAR(4)]/4. For the first ventricularinterval 620, the Stability detection enhancement calculates the averagevariance using the following weighted average formula:VAR_(AVG)(NEW)=VAR_(SEED)*Kvar+VAR(5)*(1−Kvar), where: Kvar=0.875. Foreach subsequent ventricular interval, starting after the first interval620, the Stability detection enhancement updates the average varianceusing the following weighted average formula:VAR_(avg)(NEW)=VAR_(avg)(NEW−1)*Kvar+VAR(n)*(1−Kvar), where: Kvar=0.875and n=the current interval number. As each new ventricular intervalvariance is summed into the weighted average, the effect of precedingventricular intervals on the average variance is diminished. The averagevariance is updated throughout the cardiac episode. If the averagevariance is greater that the applicable stability threshold, then theinterval is considered unstable.

When the duration time interval is satisfied, the Stability of theventricular interval is evaluated by comparing the current averagevariance to a programmed stability threshold value. If the averagevariance is equal to or greater than the programmed Stability thresholdvalue, the ventricular rate is declared unstable by the system 20.Stability threshold values are programmable within the range about from6 to 120 ms, where 22 ms is a suitable value. The Stability detectionenhancement can also be programmed OFF. The Stability detectionenhancement is used in conjunction with the atrial fibrillationdetection enhancement when it is enabled in the same zone of amulti-zone configuration.

Referring again to FIG. 4, when the stability detection enhancement isprogrammed as a therapy inhibitor and an unstable ventricular rhythm isdetected, the system 20 inhibits delivering therapy to the heart 30until the SRD time-period has expired. This is useful for ventricularrhythms originating in the atrium that may appear unstable in theventricle when the rate exceeds the lowest rate threshold, but thephysician does not intend for those rhythms to be treated by the pulsegenerator. If inhibited, reevaluation for stability continues on eachnew detected interval. Evaluation for stability continues for as long asthe detection window remains satisfied or until the SRD time has expired(if programmed on). If the ventricular rate becomes stable, treatment isinitiated immediately. The enhancement is evaluated for each ventricularinterval after the end of duration time interval until the stabilityenhancement is no longer met (i.e., the ventricular rate becomes stable)or the SRD time-period timer expires. Programming the Stabilitydetection enhancement to inhibit delivering therapy if the ventricularrate is unstable is limited to the lowest zone of a two- or three-zoneconfiguration. The Stability detection enhancement set to inhibit mayalso be used in conjunction with the SRD time-period.

Alternatively, the Stability detection enhancement can be programmed todeliver the first shock therapy if the ventricular rhythm determined tobe unstable. When the system 20 is programmed to deliver shock therapyif the ventricular rhythm is unstable, the Stability detectionenhancement determines if the remaining ATP therapy should be skippedover in preference of the first programmed shock therapy (which may below or high energy) for the satisfied rate zone. Dynamic ventriculararrhythmias such as polymorphic VT or VF (that generally respond best toshock therapy) may be sensed at a rate lower than the highest ratethreshold and may be sensed as unstable. Since the sensed rhythm may bedetected in a lower zone in which ATP may be programmed, the Stabilityparameter may be used to skip over the programmed ATP therapies andinstead provide shocks to the patient. Stability is evaluated on eachdetected ventricular cycle, including evaluation between pacing burstsof an ATP scheme. Once a shock has been delivered in an episode, thisStability function (programmed to shock if unstable) no longer affectstherapy selection. The Stability detection enhancement programmed inthis manner can be used in the VT zone of a two-zone configuration orthree zone configuration. It cannot be programmed in this manner in atwo zone configuration if the Stability detection enhancement is alreadyprogrammed to inhibit delivering therapy if the ventricular rate isunstable or it the Onset rate detection enhancement is programmed on.

Referring to FIG. 4, the Onset rate and Stability detection enhancementsprogrammed as inhibitors may be combined to provide greater specificityin characterizing ventricular tachycardias which should not be treatedby the system 20. The two detection enhancements can be programmed suchthat to initiate therapy, Onset rate AND Stability must be satisfied450, or such that either Onset rate or Stability must be satisfied 460.If the combination programmed is Onset rate OR Stability, therapy isinitiated immediately at the end of duration if either parameter issatisfied; that is, the rhythm is sudden or stable (the OR condition issatisfied). If the combination programmed is Onset rate AND Stability,therapy is initiated only if parameters are satisfied; that is, therhythm is sudden and stable (the AND condition is satisfied). When thesetwo combinations (AND/OR) are used in conjunction with the SRDtime-period, and the and/or conditions are not satisfied, therapy willbe inhibited until the SRD time-period times out. The Onset rate andStability inhibitors is limited to the lowest zone of a multiple ratezone configuration.

If AFib rate threshold/AFib stability and Onset rate are both programmedon, the combination is an AND condition. This is, to initiate therapythe rhythm must have a sudden Onset rate and either the ventricular ratemust be stable or the atrial rate must be less than the AFib ratethreshold.

Monomorphic VT is an arrhythmia which is likely to be pace terminable,and it is sensed as highly stable from beat to beat. In contrast, AF,polymorphic VT, and VF are characterized by erratic or unstable sensedrates in the ventricle, and are not generally pace terminable usingventricular pacing. When selected by the user and an unstable rhythm isdeclared, any programmed anti-tachycardia pacing therapies is bypassedin order to deliver shock therapy. Once a shock has been delivered in anepisode the stability as an accelerator function no longer affecttherapy selection. Stability as an accelerator is evaluated only once atthe expiration of the duration time interval and is used in the VT ratezone of a multi-zone configuration.

Referring now to FIG. 7, there is shown an example of the use of thedetection enhancements according to the present invention. The system 20is initially programmed with a three zone configuration, where the VF,VT, and VT-1 rate zones are enabled. The lower rate threshold for the VFrate zone is programmed to 250 bpm, for the VT rate zone it isprogrammed to 180, and for the VT-1 rate zone it is programmed to 120bpm. The programmed atrial fibrillation rate threshold value isprogrammed to 250 bpm.

The systems 20 is programmed to detect ventricular intervals in ashifting series of 10 of the most recent consecutive ventricularR-waves. As previously discussed this shifting sampling of ventricularintervals is called a detection window. The system 20 assess the sensedventricular interval in relation to each of the rate zones, where aseach new ventricular interval rate is measured by the system 20 it iscompared to each rate zone's lower rate threshold. The ventricularinterval rates are classified as being either a fast or a slowventricular interval with respect to each of the rate zones. For thisexample, the predetermined percentage of fast ventricular interval rateswithin a detection window necessary to satisfy a rate zone detectionwindow is programmed to eighty percent, or 8 of the 10 most recentconsecutive ventricular R-waves.

By way of this example, it is assumed that the patient's ventricularrate has increased from 85 bpm to 150 bpm between a first ventricularinterval 700 and a second ventricular interval 702. The system 20continues to monitor and classify each detected ventricular interval aseither fast or slow with respect to each programmed rate zone. Thesystem 20 becomes “satisfied” that the ventricular rate intervals forthe heart are properly classified in the VT-1 rate zone at 704, as 6 of10 fast ventricular beats have fallen within the VT-1 rate zone. Thesystem 20 then begins the duration time interval and continues tomonitor the ventricular interval rates within the shifting detectionwindow to be sure that the ventricular rate interval for the heartremains properly classified in the VT-1 rate zone. The maintenancepercentage for the ventricular interval rates in the subsequentdetection windows is programmed to be 60 percent, or 6 of the 10 mostrecent consecutive ventricular R-waves.

The duration time interval for the VF rate zone is programmed to 1seconds, for the VT rate zone it is programmed to 1 seconds, and for theVT-1 rate zone it is programmed to 2.5 seconds. FIG. 7 shows a selectedregion of sensed ventricular and atrial intervals surrounding theventricular interval which causes the VT-1 rate zone to become satisfiedat 704, and it is assumed for this example that the VT-1 rate zonedetection window remains satisfied throughout the duration of it'sduration time interval time, and the higher rate zones (i.e., the VT andVF rate zones) do not become satisfied.

The ventricular tachycardia therapy for the VT-1 rate zone is programmedto first deliver an ATP burst therapy then proceed to a ATP ramp therapyfor its ATP therapy prescriptions. Five cardioverting/defibrillationshocks are programmable into the system 20 for the VT-1 rate zone, wherethe first cardioverting or defibrillation shock is programmed at 5joules and the second cardioverting or defibrillation shock isprogrammed at 10 joules. As previously mentioned, the last threepotential cardioverting/defibrillation shocks are given at the maximumshock output of 29 joules.

Once the detection window contains three fast consecutive ventricularbeats, the system begins to monitor the atrial intervals and begins toanalyze the programmed detection enhancements. All four detectionenhancements (V>A+B, atrial fibrillation detection, Onset rate, andStability) and the SRD time-period are programmed on, with the Stabilityand the atrial fibrillation detection enhancements being programmed as atherapy inhibitor. The bias factor, B, is programmed with a value of 10bpm and the SRD time-period is programmed with a duration value of 5minutes. As previously discussed, if a detection enhancement as aninhibitor (i.e., the Stability detection enhancement for the VT-1 ratezone) is inhibiting therapy and the detection window is remainingsatisfied for the VT-1 rate zone, the SRD time-period begins at the endof the VT-1 rate zone duration time interval. If the detection windowcontinues to remain satisfied during the SRD time-period, the programmedtherapy will be delivered at the end of the SRD time-period.

Referring again to FIG. 7, upon determining that the heart has satisfiedthe duration time interval for the VT-1 rate zone at 706, the system 20simultaneously analyzes all four programmed detection enhancements. Thesystem 20 has detected and analyzed the atrial rate to be 545 bpm andthe ventricular rate to be 150 bpm. The V>A+B detection enhancement is,therefore, not satisfied and therapy with respect to the V>A+B detectionenhancement is inhibited. Concurrent with this assessment, the system 20also utilizes the Onset rate detection enhancement to calculate a pivotpoint as previously discussed. By way of this example it is assumed thatthe system determines that the pivot point decrease is sudden accordingto the procedure described above, therefore, stage one of the Onset ratedetection enhancement is satisfied. During the second stage of the Onsetrate assessment, the baseline average value is compared to the value ofthe pivot point interval and each of the subsequent three ventricularintervals, where, for the example, the system 20 determines that thedifference between the baseline average and the pivot point interval andthe three ventricular intervals is greater than or equal to the onsetthreshold value. The onset is, therefore, determined to be sudden.

The ventricular stability is also assessed concurrently with the V>A+Band Onset rate detection enhancements by the Stability detectionenhancement. Stability is assessed as previously discussed and thesystem 20 determines that the ventricular rate is unstable. Once theVT-1 duration time interval has expired, the system 20 examines andclassifies the 10 most recent atrial intervals as being either shorteror longer than the programmed atrial fibrillation rate threshold value.If a majority number of the first set of atrial intervals (6 of the last10 atrial intervals) are classified as being shorter than the atrialfibrillation rate threshold value AFib, the atrial rhythm is declared tobe atrial fibrillation. FIG. 7 indicates that the 10 most recent atrialintervals after the VT-1 duration time interval expired occurred over a1100 millisecond interval, which is a atrial rate of 545 bpm indicatingthe presence of an atrial fibrillation. As a result of the atria beingin a state of fibrillation and the ventricular interval rate beingdetermined to be both sudden and unstable, the system 20 inhibitsdelivering therapy at the expiration of the VT-1 rate zone duration timeinterval until either the SRD time-period has expired, until theventricular interval rate becomes stable, or until the atrialfibrillation criterion are no longer satisfied. Alternatively, if therate accelerates to a higher rate zone and the duration time intervalfor the higher rate zone is satisfied, therapy is initiated withoutwaiting for SRD time period to time out.

1-24. (canceled)
 25. An implantable cardioverter-defibrillator,comprising: an atrial catheter including at least one atrial sensingelectrode; a ventricular catheter including at least one ventricularsensing electrode and at least one ventricular defibrillation electrode;sensing circuitry, coupled to the atrial catheter and the ventricularcatheter, the sensing circuitry adapted to sense atrial events andventricular events from the electrodes; therapy delivery circuitry,coupled to the ventricular catheter, the therapy delivery circuitryadapted to apply or inhibit a ventricular tachycardia therapy; andcontrol circuitry, coupled to the sensing circuitry and therapycircuitry, the control circuitry adapted to: detect at least one of aventricular tachycardia and a ventricular fibrillation by analyzing thesensed ventricular events; start a first time interval when the at leastone of the ventricular tachycardia and the ventricular fibrillation isdetected; and analyze the sensed atrial and ventricular events duringthe first time interval for assessing an origin of the at least one of aventricular tachycardia and a ventricular fibrillation to determinewhether to apply or inhibit the ventricular tachycardia therapy.
 26. Theimplantable cardioverter-defibrillator of claim 25, wherein the controlcircuitry is adapted to: detect the at least one of the ventriculartachycardia and the ventricular fibrillation during the first timeinterval; and reset the first time interval if the at least one of theventricular tachycardia and the ventricular fibrillation is detectedduring the first time interval.
 27. The implantablecardioverter-defibrillator of claim 25, wherein the control circuitry isadapted to: determine an average atrial rate from the sensed atrialevents; determine an average ventricular rate from the sensedventricular events; and determine whether the average ventricular rateis not greater than the average atrial rate by at least a bias factor.28. The implantable cardioverter-defibrillator of claim 27, wherein thetherapy delivery circuitry is adapted to inhibit the ventriculartachycardia therapy if the average ventricular rate is not greater thanthe average atrial rate by at least the bias factor.
 29. The implantablecardioverter-defibrillator of claim 25, wherein the control circuitry isadapted to determine whether an onset rate is gradual during the firsttime interval, wherein the onset rate is the rate of transition of aventricular rate from a slower sinus rate to a tachycardia rate and theonset rate is gradual when the onset rate is equal to or greater than anonset threshold value.
 30. The implantable cardioverter-defibrillator ofclaim 29, wherein the therapy delivery circuitry is adapted to inhibitthe ventricular tachycardia therapy when the onset rate is gradual. 31.The implantable cardioverter-defibrillator of claim 25, wherein thecontrol circuitry is adapted to determine whether the ventricular rateis unstable, wherein the ventricular rate is unstable when an averagevariance of ventricular intervals exceeds a stability interval thresholdvalue.
 32. The implantable cardioverter-defibrillator of claim 31,wherein the therapy delivery circuitry is adapted to inhibit theventricular tachycardia therapy when the ventricular rate is unstable.33. The implantable cardioverter-defibrillator of claim 31, wherein thecontrol circuitry is adapted to determine an occurrence of atrialfibrillation from the sensed atrial events.
 34. The implantablecardioverter-defibrillator of claim 33, wherein the therapy deliverycircuitry is adapted to inhibit the ventricular tachycardia therapy ifthe ventricular rate is unstable and atrial fibrillation is occurring.35. A method, comprising: sensing atrial and ventricular eventsrepresentative of cardiac rhythms; detecting at least one of aventricular tachycardia and a ventricular fibrillation; analyzing thecardiac rhythms during a first time interval for assessing an origin ofthe at least one of a ventricular tachycardia and a ventricularfibrillation, the first time interval started when the at least one of aventricular tachycardia and a ventricular fibrillation is detected; anddelaying or inhibiting a ventricular tachycardia therapy if the originof the at least one of a ventricular tachycardia and a ventricularfibrillation is in an atrium.
 36. The method of claim 35, whereinanalyzing the cardiac rhythms includes: determining an average atrialrate from the sensed atrial events and determining an averageventricular rate from sensed ventricular events; and determining whetherthe average ventricular rate is not greater than the average atrial rateby at least a bias factor during the first time interval.
 37. The methodof claim 35, wherein analyzing the cardiac rhythms includes: determininga ventricular rate from sensed ventricular events; determining an onsetrate, the onset rate being the rate of transition of a ventricular ratefrom a slower sinus rate to a tachycardia rate; and determining whetherthe onset rate is equal to or greater than an onset threshold value. 38.The method of claim 35, wherein analyzing the cardiac rhythms includes:determining a ventricular rate from sensed ventricular events; anddetermining whether the ventricular rate is unstable, wherein theventricular rate is unstable when an average variance of ventricularintervals exceeds a stability interval threshold value.
 39. The methodof claim 38, wherein analyzing the cardiac rhythms further includesdetermining an occurrence of atrial fibrillation from the sensed atrialevents.
 40. A system, comprising: means for sensing atrial events andventricular events; means for determining ventricular interval ratesfrom pairs of consecutively sensed ventricular events in a detectionwindow; means for declaring the detection window satisfied when a firstpredetermined percentage of the ventricular interval rates in thedetection window are classified as fast ventricular intervals; means forstarting a first time interval programmed for a duration associated withthe ventricular interval rates once the detection window is declaredsatisfied; and means for determining whether to inhibit a ventriculartachycardia therapy at the end of the first time interval.
 41. Thesystem of claim 40, wherein the means for determining whether to inhibitthe ventricular tachycardia therapy comprises means for determiningwhether the fast ventricular intervals sustain during the first timeinterval.
 42. The system of claim 41, wherein the means for determiningwhether the fast ventricular intervals sustain comprises: means fordeclaring that the detection window remains satisfied when a secondpredetermined percentage of the ventricular interval rates in thedetection window are classified as fast ventricular intervals; and meansfor resetting the first time interval to zero when the detection windowfails to remain satisfied during the first time interval.
 43. The systemof claim 41, wherein the means for determining whether to inhibit theventricular tachycardia therapy comprises means for determining theorigin of the fast ventricular intervals during the first time interval.